Phosphor, Radiation image recording and reproducing method and radiation image storage panel

ABSTRACT

A cerium activated rare earth halide phosphor having the formula (I): 
     
         LnX.sub.3 ·aM.sup.I X&#39;:xCe.sup.3+                 (I) 
    
     in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; M I  is at least one alkali metal selected from the group consisting of Li, Na, K, Cs and Rb; each of X and X&#39; is at least one halogen selected from the group consisting of Cl, Br and I; and a and x are numbers satisfying the conditions of O&lt;a≦10.0 and O&lt;x≦0.2, respectively. A process for the preparation of said phosphor, a radiation image recording and reproducing method utilizing said phosphor, and a radiation image storage panel employing said phosphor are also disclosed.

This application is a continuation of Ser. No. 753,541, filed July 10,1985, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel phosphor, a process for thepreparation of the same, a radiation image recording and reproducingmethod utilizing the same, and a radiation image storage panel employingthe same. More particularly, the invention relates to a novel ceriumactivated rare earth halide phosphor.

2. Description of the Prior Art

There is well known a cerium activated rare earth oxyhalide phosphor(LnOX:Ce, in which Ln is at least one rare earth element selected fromthe group consisting of Y, La, Gd and Lu; and X is at least one halogenselected from the group consisting of Cl and Br) as a cerium activatedrare earth halide phosphor. As described in Japanese Patent ProvisionalPublication No. 55(1980)-12144 (which corresponds to U.S. Pat. No.4,236,078), etc., the phosphor gives emission (stimulated emission) inthe near ultraviolet region when excited with an electromagnetic wavesuch as visible light or infrared rays after exposure to a radiationsuch as X-rays, cathode rays or ultraviolet rays. The phosphor isvaluable as a stimulable phosphor employable for a radiation imagerecording and reproducing method.

The radiation image recording and reproducing method utilizing thestimulable phosphor can be employed in place of the conventionalradiography utilizing a combination of a radiographic film having anemulsion layer containing a photosensitive silver salt and anintensifying screen as described, for instance, in U.S. Pat. No.4,239,968. The method involves steps of causing a stimulable phosphor toabsorb a radiation having passed through an object or having radiatedfrom an object; sequentially exciting (or scanning) the phosphor with anelectromagnetic wave such as visible light or infrared rays (stimulatingrays) to release the radiation energy stored in the phosphor as lightemission (stimulated emission); photoelectrically detecting the emittedlight to obtain electric signals; and reproducing the radiation image ofthe object as a visible image from the electric signals.

In the radiation image recording and reproducing method, a radiationimage is obtainable with a sufficient amount of information by applyinga radiation to the object at a considerably smaller dose, as comparedwith the conventional radiography. Accordingly, this method is of greatvalue, especially when the method is used for medical diagnosis.

For other stimulable phosphors employable in the above-described method,there have been known a divalent europium activated alkaline earth metalfluorohalide phosphor (M^(II) FX:Eu²⁺, in which M^(II) is at least onealkaline earth metal selected from the group consisting of Mg, Ca andBa; and X is at least one halogen selected from the group consisting ofCl, Br and I); an europium and samarium activated strontium sulfidephosphor (SrS: Eu,Sm); an europium and samarium activated lanthanumoxysulfide phosphor (La₂ O₂ S:Eu,Sm); an europium activated bariumaluminate phosphor (BaO.Al₂ O₃ :Eu) an europium activated alkaline earthmetal silicate phosphor (M²⁺. SiO₂ :Eu, in which M²⁺ is at least onealkaline earth metal selected from the group consisting of Mg, Ca andBa), and the like.

SUMMARY OF THE INVENTION

The present invention provides a novel cerium activated rare earthhalide phosphor which is different from the above-mentioned ceriumactivated rare earth oxyhalide phosphor, and a process for thepreparation of the same.

The present invention has researched for a stimulable phosphor and newlyfound that a cerium activated rare earth halide phosphor givesstimulated emission as well as spontaneous emission, to accomplish theinvention.

The phosphor of the invention is a cerium activated rare earth halidephosphor having the formula (I):

    LnX.sub.3 ·aM.sup.I X':xCe.sup.3+                 (I)

in which Ln is at least one rare earth element selected from the groupconsisting of Y, La, Gd and Lu; M^(I) is at least one alkali metalselected from the group consisting of Li, Na, K, Cs and Rb; each of Xand X' is at least one halogen selected from the group consisting of Cl,Br and I; and a and x are numbers satisfying the conditions of 0 <a≦10.0 and 0>×≦0.2, respectively.

The process for the preparation of the phosphor having the formula (I)of the invention comprises:

mixing starting materials for the phosphor in a stoichiometric ratiocorresponding to the formula (II):

    LnX.sub.3 ·aM.sup.I X':xCe                        (II)

in which Ln, M^(I), X, X', a and x have the same meanings as definedabove; and firing the obtained mixture at a temperature within the rangeof 500°-1300° C. in a weak reducing atmosphere.

The cerium activated rare earth halide phosphor having the formula (I)of the invention gives stimulated emission in the near ultraviolet toblue region when excited with an electromagnetic wave having awavelength within the range of 500-850 nm after exposure to a radiationsuch as X-rays, ultraviolet rays and cathode rays.

The cerium activated rear earth halide phosphor having the formula (I)of the invention also gives emission (spontaneous emission) in the nearulatraviolet to blue region when exposed to a radiation such as X-rays,ultraviolet rays and cathode rays.

The present invention further provides a radiation image recording andreproducing method utilizing the novel stimulable phosphor and aradiation image storage panel using said phosphor.

That is, the radiation image recording and reproducing method comprisessteps of:

(i) causing the cerium activated rare earth halide phosphor having theformula (I) to absorb a radiation having passed through an object orhaving radiated from an object;

(ii) exciting said stimulable phosphor with an electromagnetic wavehaving a wavelength within the range of 500-850 nm to release theradiation energy stored therein as light emission; and

(iii) detecting the emitted light.

The radiation image storage panel of the invention comprises a supportand stimulable phosphor layer provided thereon which comprises a binderand a stimulable phosphor dispersed therein, in which said phosphorlayer contains the cerium activated rare earth halide phosphor havingthe formula (I).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows spontaneous emission spectra of LaBr₃. CsCl:0.001Ce³⁺phosphor and LaCl₃ ·CsBr:0.001Ce³⁺ phosphor and excitation spectrathereof (Curves 1, 2, 3 and 4, respectively), which are examples of thecerium activated rare earth halide phosphor according to the invention.

FIG. 2 shows a stimulation spectrum of the LaBr₃. CsCl:0.001Ce³⁺phosphor.

FIG. 3 shows stimulated emission spectra of the LaBr₃ ·CsCl:0.001Ce³⁺phosphor and the LaCl₃ ·CsBr: 0.001Ce³⁺ phosphor (Curves 1 and 2,respectively).

FIG. 4 shows a relationship between a value and an intensity ofstimulated emission with respect to LaBr₃ ·aCsCl:0.001Ce³⁺ phosphor,which is an example of the cerium activated rare earth halide phosphoraccording to the invention.

FIG. 5 is a schematic view showing the radiation image recording andreproducing method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The cerium activated rare earth halide phosphor of the present inventioncan be prepared, for instance, by a process described below.

As starting materials, the following materials can be employed:

(1) at least one rare earth halide selected from the group consisting ofYCl₃, YBr₃, YI₃, LaCl₃ LaBr₃, LaI₃, GdCl₃, GdBr₃ GdI₃, LuCl₃, LuBr₃ andLuI₃ ;

(2) at least one alkali metal halide selected from the group consistingof LiCl, LiBr, LiI, NaCl, NaBr, NaI, KCl, KBr, KI, CsCl, CsBr, CsI,RbCl, RbBr and RbI; and

(3) at least one compound selected from the group consisting of ceriumcompounds such as cerium halide, cerium oxide, cerium nitrate and ceriumsulfate.

Further, ammonium halide (NH₄ X", in which X" is any one of Cl, Br andI) may be employed as a flux.

In the process for the preparation of the phosphor of the invention, theabove-mentioned rare earth halide (1), alkali metal halide (2) andcerium compound (3) are, in the first place, mixed in the stoichiometricratio corresponding to the formula (II):

    LnX.sub.3 ·aM.sup.I X':xCe                        (II)

in which Ln is at least one rare earth element selected from the groupconsisting of Y, La, Gd and Lu; M^(I) is at least one alkali metalselected from the group consisting of Li, Na, K, Cs and Rb; each of Xand X' is at least one halogen selected from the group consisting of Cl,Br and I; and a and x are numbers satisfying the conditions of 0 <a≦10.0and 0<≦0.2, respectively.

From the viewpoint of enhancement in the intensity of stimulatedemission and in the intensity of spontaneous emission, Ln in the formula(II) which indicates rare earth element is preferably at least oneelement selected from the group consisting of Y and La. M^(I) indicatingalkali metal is preferably at least one element selected from the groupconsisting of Cs and Rb. Each of X and X' which indicates halogen ispreferably Cl or Br, and both are preferably different from each other.The number for a which indicates the amount of alkali metal halide(M^(I) X') is preferably within the range of 0.1≦a≦2.0, and morepreferably of 0.2≦a≦1.0. From the same viewpoint, the number for x whichindicates the amount of cerium activator is preferably within the rangeof 10⁻⁵ ≦×10⁻².

The mixture of starting materials for the phosphor is prepared by anyone of the following procedures;

(i) simply mixing the starting materials (1), (2) 30 and (3);

(ii) mixing the starting materials (1) and (2), heating the obtainedmixture at a temperature of hot lower than 100° C. for several hours andthen mixing the heat-treated mixture with the starting material (3); and

(iii) mixing the starting materials (1) and (2) in the form of asolution, drying the solution by reduced pressure drying, vacuum dryingor spray drying under heating (preferably, 50°-200° C.), and then mixingthe obtained dry product with the starting material (3).

Further, as a modification of the above procedure (ii), there may bementioned a procedure comprising mixing the starting materials (1), (2)and (3) and subjecting the obtained mixture to the heating treatment. Asother modification of the procedure (iii), there may be mentioned aprocedure comprising mixing the starting materials (1), (2) and (3) inthe form of a solution and subjecting the solution to the drying.

The mixing is carried out using a conventional mixing apparatus such asa variety of mixers, a V-type blender, a ball mill and a rod mill in anycase of the above-described procedures (i), (ii) and (iii).

Then, the resulting mixture of the starting materials is placed in aheat-resistant container such as a quartz boat, an alumina crucible or aquartz crucible, and fired in an electric furnace. The temperature forthe firing suitably ranges from 500° to 1300° C., and preferably rangesfrom 700° to 1000° C.. The firing period is determined depending uponthe amount of the mixture of starting materials, the firing temperature,etc., and suitably ranges from 0.5 to 6 hours. As the firing atmosphere,there can be employed a weak reducing atmosphere such as a nitrogen gasatmosphere containing a small amount of hydrogen gas or a carbon dioxidegas atmosphere containing carbon monoxide gas. In the case of using atetravalent cerium compound as the above-mentioned starting material(3), the tetravalent cerium contained in the mixture is reduced intotrivalent cerium by the weak reducing atmosphere in the firing stage.

Through the firing procedure, a powdery phosphor of the presentinvention is produced. The powdery phosphor thus obtained may beprocessed in a conventional manner involving a variety of procedures forthe preparation of phosphors such as a washing procedure, a dryingprocedure and a sieving procedure.

The phosphor of the invention prepared in accordance with theabove-described process is a cerium activated rare earth halide phosphorhaving the formula (I):

    LnX.sub.3 ·aM.sup.I X':xCe.sup.3+                 (I)

in which Ln is at least one rare earth element selected from the groupconsisting of Y, La, Gd and Lu; M^(I) is at least one alkali metalselected from the group consisting of Li, Na, K, Cs and Rb; each of Xand X' is at least one halogen selected from the group consisting of Cl,Br and I; and a and x are numbers satisfying the conditions of 0 <a≦10.0and 0<×0.2, respectively.

The cerium activated rare earth halide phosphor of the present inventiongives spontaneous emission in the near ultraviolet to blue region (peakwavelength of the emission: approx. 360-380 nm) upon excitation with aradiation such as X-rays, ultraviolet rays and cathode rays.

FIG. 1 shows examples of spontaneous emission spectra of ceriumactivated rare earth halide phosphors according to the invention andexcitation spectra thereof;

Curve 1: spontaneous emission spectrum of LaBr₃ ·CsCl:0.001Ce³⁺phosphor;

Curve 2: spontaneous emission spectrum of LaCl₃ ·CsBr:0.001Ce³⁺phosphor;

Curve 3: excitation spectrum of LaBr₃.CsCl:0.001Ce³⁺ phosphor; and

Curve 4: excitation spectrum of LaCl₃ ·CsBr:0.001Ce³⁺ phosphor.

As is clear from FIG. 1, the phosphors according to the invention givespontaneous emission in the near ultraviolet to blue region uponexcitation with ultraviolet rays. The peak of the emission spectrum is365 nm for LaBr₃ ·CsCl:0.001Ce³ phosphor, and 375 nm for LaCl₃.CsBr:0.001Ce³⁺ phosphor.

The spontaneous emission spectra upon excitation with ultraviolet raysand excitation spectra of the cerium activated rare earth halidephosphors of the invention are illustrated above, for the two kinds ofphosphors. It has been confirmed that spontaneous emission spectra andexcitation spectra of other phosphors according to the invention aresimilar to those of the above-stated two kinds of phosphors. Also hasbeen confirmed that the spontaneous emission spectrum of the phosphor ofthe invention given upon excitation with X-rays or cathode rays arealmost the same as those given upon excitation with ultraviolet rayswhich are shown in FIG. 1.

The cerium activated rear earth halide phosphor of the invention alsogives stimulated emission in the near ultraviolet to blue region whenexcited with an electromagnetic wave having a wavelength within theregion of 500-850 nm such as visible light or infrared rays afterexposure to a radiation such as X-rays, ultraviolet rays and cathoderays.

FIG. 2 shows a stimulation spectrum of LaBr₃ ·CsCl:0.001Ce³⁺ phosphorwhich is an example of cerium activated rare earth halide phosphor ofthe invention.

As is clear from FIG. 2, the phosphor of the invention give stimulatedemission upon excitation with an electromagnetic wave in the wavelengthregion of 500-850 nm after exposure to X-rays. Particularly, thephosphor exhibits stimulated emission of high intensity upon excitationwith an electromagnetic wave in the wavelength region of 500-700 nm.Based on this fact, the wavelength region of the electromagnetic waveemployed as stimulating rays, namely 500-850 nm, has been decided in theradiation image recording and reproducing method of the presentinvention.

FIG. 3 shows examples of stimulated emission spectra of the ceriumactivated rare earth halide phosphors according to the invention:

Curve 1: stimulated emission spectrum of LaBr₃ ·CsCl:0.01Ce³⁺ phosphor;and

Curve 2: stimulated emission spectrum of LaCl₃ ·CsBr:0.001Ce³⁺ phosphor.

As is clear from FIG. 3, the phosphors according to the invention givestimulated emission in the near ultraviolet to blue region, and eachpeak wavelength of the emission spectra is within the range of approx.360-380 nm. The stimulated emission spectra of the phosphors are in goodaccordance with the spontaneous emission spectra thereof shown in FIG.1.

The stimulated emission spectra and stimulation spectra of the ceriumactivated rate earth phosphors according to the present invention areillustrated above with respect to the specific phosphors. It has beenconfirmed that other phosphors according to the invention show thesimilar stimulated emission characteristics to those of theabove-mentioned specific phosphors, and further confirmed that they givestimulated emission in the near ultraviolet to blue region when excitedwith an electromagnetic wave having a wavelength within the range of500-850 nm after exposure to a radiation and each peak wavelength iswithin the range of approx. 360-380 nm.

FIG. 4 graphically shows a relationship between a value and an intensityof stimulated emission [emission intensity upon excitation with a He-Nelaser (wavelength: 632.8 nm) after exposure to X-rays at 80 KVp]withrespect to LaBr₃ ·aCsCl:0.001Ce³⁺ phosphor.

As is evident from FIG. 4, the LaBr₃ ·aCsCl:0.001Ce³⁺ phosphor having avalue within a range of 0<a≦10.0 gives stimulated emission. On the basisof this fact, the a value range (0<a≦10.0) of the phosphor of theinvention has been decided. Particularly, the emission intensity of thephosphor is high in the a value range of 0.1≦a≦2.0, and is further highin the range of 0.2≦a≦1.0.

The phosphor has almost the same tendency as shown in FIG. 4 withrespect to the relationship between a value and an intensity ofspontaneous emission. It has been further confirmed that other ceriumactivated rare earth halide phosphors according to the invention thanthe above-mentioned phosphor have the same tendencies on therelationships between a value and the intensity of stimulated emissionand between a value and the intensity of spontaneous emission as shownin FIG. 4.

From the viewpoint of emission properties described hereinbefore, thephosphor of the invention is very useful as a phosphor for a radiationimage storage panel employed in the radiation image recording andreproducing method, or for a radiographic intensifying screen employedin the conventional radiography, both panel and screen being used inmedical radiography such as X-ray photography for medical diagnosis andindustrial radiography for non-destructive inspection.

The cerium activated rare earth halide phosphor having the formula (I)is preferably employed in the form of a radiation image storage panel(also referred to as a stimulable phosphor sheet) in the radiation imagerecording and reproducing method of the invention.

The radiation image storage panel comprises a support and at least onephosphor layer provided on one surface of the support. The phosphorlayer comprises a binder and a stimulable phosphor dispersed therein.Further, a transparent protective film is generally provided on the freesurface of the phosphor layer (surface not facing the support) to keepthe phosphor layer from chemical deterioration or physical shock.

The radiation image recording and reproducing method of the invention isdesired to be performed employing the radiation image storage panelcomprising a phosphor layer which contains the cerium activated rareearth halide phosphor having the formula (I).

In the radiation image recording and reproducing method employing thestimulable phosphor having the formula (I) in the form of a radiationimage storage panel, a radiation having passed through an object orradiated from an object is absorbed by the phosphor layer of the panelto form a radiation image as a radiation energy-stored image on thepanel. The panel is then irradiated (e.g., scanned) with anelectromagnetic wave in the wavelength region of 500-850 nm to releasethe stored image as stimulated emission. The emitted light isphotoelectrically detected to obtain electric signals so that theradiation image of the object can be reproduced as a visible image fromthe obtained electric signals.

The radiation image recording and reproducing method of the presentinvention will be described more in detail with respect to an example ofa radiation image storage panel containing the stimulable phosphorhaving the formula (I), by referring to a schematic view shown in FIG.5.

In FIG. 5 which shows the total system of the radiation image recordingand reproducing method of the invention, a radiation generating device11 such as an X-ray source provides a radiation for irradiating anobject 12 therewith; a radiation image storage panel 13 containing thestimulable phosphor having the formula (I) absorbs and stores theradiation having passed through the object 12; a source of stimulatingrays 14 provides an electromagnetic wave for releasing the radiationenergy stored in the panel 13 as light emission; a photosensor 15 suchas a photomultiplier faces the panel 13 for detecting the light emittedby the panel 13 and converting it to electric signals; an imagereproducing device 16 is connected with the photosensor 15 to reproducea radiation image from the electric signals detected by the photosensor15; a display device 17 is connected with the reproducing device 16 todisplay the reproduced image in the form of a visible image on a CRT orthe like; and a filter 18 is disposed in front of the photosensor 15 tocut off the stimulating rays reflected by the panel 13 and allow onlythe light emitted by the panel 13 to pass through.

FIG. 5 illustrates an example of the system according to the method ofthe invention employed for obtaining a radiation-transmission image ofan object. However, in the case that the object 12 itself emits aradiation, it is unnecessary to install the above-mentioned radiationgenerating device 11. Further, the photosensor 15 to the display device17 in the system can be replaced with other appropriate devices whichcan reproduce a radiation image having the information of the object 12from the light emitted by the panel 13.

Referring to FIG. 5, when the object 12 is exposed to a radiation suchas X-rays provided by the radiation generating device 11, the radiationpasses through the object 12 in proportion to the radiationtransmittance of each portion of the object. The radiation having passedthrough the object 12 impinges upon the radiation image storage panel13, and is absorbed by the phosphor layer of the panel 13. Thus, aradiation energy-stored image (a kind of latent image) corresponding tothe radiation-transmission image of the object 12 is formed on the panel13.

Thereafter, when the radiation image storage panel 13 is irradiated withan electromagnetic wave having the wavelength within the range of500-850 nm, which is provided by the source of stimulating rays 14, theradiation energy-stored image formed on the panel 13 is released aslight emission. The intensity of so released light is in proportion tothe intensity of the radiation energy which has been absorbed by thephosphor layer of the panel 13. The light signals corresponding to theintensity of the emitted light are converted to electric signals bymeans of the photosensor 15, the electric signals are reproduced as animage in the image reproducing device 16, and the reproduced image isdisplayed on the display device 17.

The operation of reading out the image information stored in theradiation image storage panel is generally carried out by sequentiallyscanning the panel with a laser beam and detecting the light emittedunder the scanning with a photosensor such as photomultiplier through anappropriate light guiding means to obtain electric signals. In order toobtain a well-readable visible image, the read-out operation maycomprise a preliminary read-out operation and a final read-outoperation, in which the panel is twice irradiated with stimulating raysthough the energy of the stimulating rays in the former is lower thanthat in the latter (see: U.S. patent application Ser. No. 434,886). Theread-out condition in the final read-out operation can be suitably setbased on the result obtained by the preliminary read-out operation.

As the photosensor, solid-state photoelectric conversion devices such asa photoconductor and a photodiode can be also used (see: U.S. patentapplication Ser. No. 610,582, Japanese Patent Applications No.58(1983)-219313 and No. 58(1983)-219314, and Japanese Patent ProvisionalPublication No. 58(1983)-121874). For example, the photosensor isdivided into a great number of pixels, which may be combined with aradiation image storage panel or positioned in the vicinity of thepanel. Otherwise, the photosensor may be a linesenor in which pluralpixels are linearly connected or may be such one that corresponds to onepixel.

In the above-mentioned cases, there may be employed for the source ofstimulating rays a linear light source such as an array in which lightemitting diodes (LED), semiconductor lasers or the like are linearlyarranged, in addition to a point light source such as a laser. Theread-out using such photosensor can prevent loss of the light emitted bya panel and can bring about the enhancement of S/N ratio of the image,because the photosensor can receive the emitted light with a largeangle. It is also possible to enhance the read-out speed, becauseelectric signals are sequentially obtained not by scanning the panelwith stimulating rays, but by electrical processing of the photosensor.

After reading out the image information stored in a radiation imagestorage panel, the panel is preferably subjected to a procedure oferasing the radiation energy remaining therein, that is, to the exposureto light having a wavelength in the wavelength region of stimulatingrays for the phosphor contained therein or to heating (see: U.S. Pat.No. 4,400,619 and Japanese Patent Provisional Publication No.56(1981)-12599). The erasing procedure can prevent the occurrence ofnoise originating from the after image in the next use of the panel.Further, the panel can be more effectively prevented from the occurrenceof noise attributable to natural radiations by carrying out the erasingprocedure twice, namely after the read-out and just before the next use(see: U.S. patent application Ser. No. 338,734).

In the radiation image recording and reproducing method of the presentinvention, there is no specific limitation on the radiation employablefor exposure of an object to obtain a radiation transmittance imagethereof, as far as the above-described phosphor gives stimulatedemission upon excitation with the electromagnetic wave after exposure tothe radiation. Examples of the radiation employable in the inventioninclude those generally known, such as X-rays, cathode rays andultraviolet rays. Likewise, there is no specific limitation on theradiation radiating from an object for obtaining a radiation imagethereof, as far as the radiation can be absorbed by the above-describedphosphor to serve as an energy source for producing the stimulatedemission. Examples of the radiation include γ-rays, α-rays and β-rays.

As the source of stimulating rays for exciting the phosphor which hasabsorbed the radiation having passed through or radiated from theobject, there can be employed, for instance, light sources providinglight having a band spectrum distribution in the wavelength region of500-850 nm; and light sources providing light having a single wavelengthor more in said region such as an Ar ion laser, a Kr ion laser, a He-Nelaser, a ruby laser, a semiconductor laser, a glass laser, a YAG laser,a dye laser and a light emitting diode (LED). Among these sources ofstimulating rays, the lasers are preferred because the radiation imagestorage panel is exposed thereto with a high energy density per unitarea. Particularly preferred are the Ar ion laser, He-Ne laser and Krion laser, from the viewpoints of the stability and output powerthereof. The semiconductor laser is also preferred, because its size issmall, it can be driven by a weak electric power and its output powercan be easily stabilized owing to the direct modulation thereof.

As the light source for erasing the radiation energy remaining in theradiation image storage panel, a light source at least providing lightof a wavelength within the wavelength region of stimulating rays for theabove-mentioned phosphor is employed. Examples of the light sourceemployable in the method of the present invention include a fluorescentlamp, a tungsten lamp and a halogen lamp.

The recording and read-out of a radiation image in the method of theinvention can be carried out by using a built-in type radiation imageconversion apparatus which comprises a recording section for recordingthe radiation image on the radiation image storage panel (i.e., causinga stimulable phosphor of the panel to absorb and store radiationenergy), a read-out section for reading out the radiation image recordedon the panel (i.e., exciting the phosphor with stimulating rays torelease the radiation energy as light emission), and an erasure sectionfor eliminate the radiation image remained in the panel (i.e., causingthe phosphor to release the remaining energy) (see: U.S. patentapplications Ser. No. 434,883 and 600,689). By employing such built-intype apparatus, the radiation image storage panel (or a recording mediumcontaining a stimulable phosphor) can be circularly and repeatedly usedand a number of images having a quality at a certain level can be stablyobtained. The radiation image conversion apparatus can be made socompact and light weight as to easily set and move the apparatus. It isfurther possible to move the apparatus place to place to record theradiation images for mass examinations by loading a traveling X-raydiagnosis station in the form of a vehicle like a bus with theapparatus.

The radiation image storage panel employable in the radiation imagerecording and reproducing method of the invention will be described.

The radiation image storage panel, as described hereinbefore, comprisesa support and a phosphor layer provided thereon which comprises a binderand the above-described cerium activated rare earth halide phosphorhaving the formula (I) dispersed therein.

The radiation image storage panel having such structure can be prepared,for instance, in the manner described below.

Examples of the binder to be employed in the phosphor layer include:natural polymers such as proteins (e.g., gelatin), polysaccharides (e.g.dextran) and gum arabic; and synthetic polymers such as polyvinylbutyral, polyvinyl acetate, nitrocellulose, ethylcellulose, vinylidenechloride-vinyl chloride copolymer, polyalkyl (meth)acrylate, vinylchloride-vinyl acetate copolymer, polyurethane, cellulose acetatebutyrate, polyvinyl alcohol, and linear polyester. Particularlypreferred are nitrocellulose, linear polyester, polyalkyl(meth)acrylate, a mixture of nitrocellulose and linear polyester, and amixture of nitrocellulose and polyalkyl (meth)acrylate.

The phosphor layer can be formed on a support, for instance, by thefollowing procedure.

In the first place, the stimulable phosphor particles and a binder areadded to an appropriate solvent, and then they are mixed to prepare acoating dispersion of the phosphor particles in the binder solution.

Examples of the solvent employable in the preparation of the coatingdispersion include lower alcohols such as methanol, ethanol, n-propanoland n-butanol; chlorinated hydrocarbons such as methylene chloride andethylene chloride; ketones such as acetone, methyl ethyl ketone andmethyl isobutyl ketone; esters of lower alcohols with lower aliphaticacids such as methyl acetate, ethyl acetate and butyl acetate; etherssuch as dioxane, ethylene glycol monoethylether and ethylene glycolmonomethylether; and mixtures of the above-mentioned compounds.

The ratio between the binder and the phosphor in the coating dispersionmay be determined according to the characteristics of the aimedradiation image storage panel and the nature of the phosphor employed.Generally, the ratio therebetween is within the range of from 1 :1 to1:100 (binder : phosphor, by weight), preferably from 1:8 to 1:40.

The coating dispersion may contain a dispersing agent to assist thedispersibility of the phosphor particles therein, and also contain avariety of additives such as a plasticizer for increasing the bondingbetween the binder and the phosphor particles in the phosphor layer.Examples of the dispersing agent include phthalic acid, stearic acid,caproic acid and a hydrophobic surface active agent. Examples of theplasticizer include phosphates such as triphenyl phosphate, tricresylphosphate and diphenyl phosphate; phthalates such as diethyl phthalateand dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethylglycolate and butylphthalyl butyl glycolate; and polyesters ofpolyethlyene glycols with aliphatic dicarboxylic acids such as polyesterof triethylene glycol with adipic acid and polyester of diethyleneglycol with succinic acid.

The coating dispersion containing the phosphor particles and the binderprepared as described above is applied evenly to the surface of asupport to form a layer of the coating dispersion. The coating procedurecan be carried out by a conventional method such as a method using adoctor blade, a roll coater or a knife coater.

A support material employed in the present invention can be selectedfrom those employed in the conventional radiographic intensifyingscreens or those employed in the known radiation image storage panels.Examples of the support material include plastic films such as films ofcellulose acetate, polyester, polyethylene terephthalate, polyamide,polyimide, triacetate and polycarbonate; metal sheets such as aluminumfoil and aluminum alloy foil; ordinary papers; baryta paper;resin-coated papers; pigment papers containing titanium dioxide or thelike; and papers sized with polyvinyl alcohol or the like. From theviewpoint of characteristics of a radiation image storage panel as aninformation recording material, a plastic film is preferably employed asthe support material of the invention. The plastic film may contain alight-absorbing material such as carbon black, or may contain alight-reflecting material such as titanium dioxide. The former isappropriate for preparing a high-sharpness type radiation image storagepanel, while the latter is appropriate for preparing a high-sensitivetype radiation image storage panel.

In the preparation of a known radiation image storage panel, one or moreadditional layers are occasionally provided between the support and thephosphor layer, so as to enhance the adhesion between the support andthe phosphor layer, or to improve the sensitivty of the panel or thequality of an image provided thereby. For instance, a subbing layer oran adhesive layer may be provided by coating a polymer material such asgelatin over the surface of the support on the phosphor layer side.Otherwise, a light-reflecting layer or a light-absorbing layer may beprovided by forming a polymer material layer containing alight-reflecting material such as titanium dioxide or a light-absorbingmaterial such as carbon black. In the invention, one or more of theseadditional layers may be provided.

As described in U.S. patent application Ser. No. 496,278 or EuropeanPatent Publication No. 92241, the phosphor layer-side surface of thesupport (or the surface of an adhesive layer, light-reflecting layer, orlight-absorbing layer in the case that such layers are provided on thephosphor layer) may be provided with protruded and depressed portionsfor enhancement of the sharpness of radiation image.

After applying the coating dispersion to the support as described above,the coating dispersion is then heated slowly to dryness so as tocomplete the formation of a phosphor layer. The thickness of thephosphor layer varies depending upon the characteristics of the aimedradiation image storage panel, the nature of the phosphor, the ratiobetween the binder and the phosphor, etc. Generally, the thickness ofthe phosphor layer is within the range of from 20 μm to 1 mm, preferablyfrom 50 to 500 μm.

The phosphor layer can be provided on the support by the methods otherthan that given in the above. For instance, the phosphor layer isinitially prepared on a sheet (false support) such as a glass plate,metal plate or plastic sheet using the aforementioned coating dispersionand then thus prepared phosphor layer is overlaid on the genuine supportby pressing or using an adhesvie agent.

The phosphor layer placed on the support can be in the form of a singlelayer or in the form of plural (two or more) layers. When the pluralphosphor layers are placed, at least one layer contains theaforementioned cerium activated rare earth halide phosphor having theformula (I), and the plural layers may be placed in such a manner that alayer nearer to the surface shows stimulated emission of high intensity.In any case, that is, in either the single phosphor layer or pluralphosphor layers, a variety of known stimulable phosphors are employablein combination with the above-mentioned stimulable phosphor.

Examples of the stimulable phosphor employable in combination with thestimulable phosphor of the invention include the aforementioned phosphorand the phosphors described below;

ZnS:Cu,Pb, BaO·XAl₂ O₃ :Eu, in which x is a number satisfying thecondition of 0.8≦×≦10, and M^(II) O·xSiO₂ :A, in which M^(II) is atleast one divalent metal selected from the group consisting of Mg, Ca,Sr, Zn, Cd and Ba, A is at least one element selected from the groupconsisting of Ce, Tb, Eu, Tm, Pb, Tl, Bi and Mn, and x is a numbersatisfying the condition of 0.5≦×≦2.5, as described in U.S. Pat. No.4,326,078;

(Ba_(l-x-y),Mg_(x),Ca_(y))FX:aEu²⁺, in which X is at least one elementselected from the group consisting of Cl and Br, x and y are numberssatisfying the conditions of 0<x+y≦0.6, and xy≠0, and a is a numbersatisfying the condition of 10⁻⁶ ≦a≦5×10⁻², as described in JapanesePatent Provisional Publication No. 55(1980)-12143; and

LnOX:xA, in which Ln is at least one element selected from the groupconsisting of La, Y, Gd and Lu, x is at least one element selected fromthe group consisting of Cl and Br, A is at least one element selectedfrom the group consisting of Ce and Tb, and x is a number satisfying thecondition of 0<×0.1, as described in the above-mentioned U.S. Pat. No.4,236,078.

A radiation image storage panel generally has a transparent film on afree surface of a phosphor layer to physically and chemically protectthe phosphor layer. In the panel of the invention, it is preferable toprovide a transparent film for the same purpose.

The transparent film can be provided on the phosphor layer by coatingthe surface of the phosphor layer with a solution of a transparentpolymer such as a cellulose derivative (e.g., cellulose acetate ornitrocellulose), or a synthetic polymer (e.g. polymethyl methacrylate,polyvinyl butyral, polyvinyl formal, polycarbonate, polyvinyl acetate,or vinyl chloride-vinyl acetate copolymer), and drying the coatedsolution. Alternatively, the transparent film can be provided on thephosphor layer by beforehand preparing if from a polymer such aspolyethylene terephthalate, polyethylene, polyvinylidene chloride orpolyamide, followed by placing and fixing it onto the phosphor layerwith an appropriate adhesive agent. The transparent protective filmpreferably has a thickness within the range of approximately 0.1 to 20μm.

The present invention will be illustrated by the following examples, butthese examples by no means restrict the invention.

EXAMPLE 1

To 800 ml of distilled water (H₂ O) were added 378.9 g. of lanthanumbromide (LaBr₃), 168.4 g. of cesium chloride (CsCl) and 0.172 g. ofcerium oxide (CeO₂), and they were mixed to obtain an aqueous solution.The aqueous solution was dried at 60° C. under reduced pressure for 3hours and further dried at 150° C. under vacuum for another 3 hours toobtain a mixture of starting materials for the preparation of aphosphor.

The mixture thus obtained was placed in an alumina crucible, which was,in turn, placed in a high-temperature electric furnace. The mixture wasthen fired at 900° C. for 2 hours under a carbon dioxide atmospherecontaining carbon monoxide. After the firing was complete, the cruciblewas taken out of the furnace and allowed to stand for cooling. Thus, apowdery cerium activated lanthanum bromide phosphor (LaBr₃ ·CsCl:0.001Ce³⁺) was obtained.

EXAMPLE 2

The procedure of Example 1 was repeated except for using 245.3 g. oflanthanum chloride (LaCl₃) and 213.0 g. of cesium bromide (CsBr) insteadof lanthanum bromide and cersium chloride, to obtain a powdery ceriumactivated lanthanum chloride phosphor (LaCl₃ ·CsBr:0.001Ce³⁺).

EXAMPLE 3

The procedure of Example 1 was repeated except for using 328.9 g. ofyttrium bromide (YBr₃) and 42.4 g. of lithium chloride (LiCl) instead oflanthanum bromide and cesium chloride, to obtain a powdery ceriumactivated yttrium bromide phosphor (YBr₃ ·LiCl:0.001Ce³⁺).

The phosphors prepared in Examples 1 and 2 were excited with ultravioletrays to measure spontaneous emission spectra and excitation spectra. Theresults are shown in FIG. 1.

In FIG. 1, Curves 1 to 4 correspond to the following spectra:

1: spontaneous emission spectrum of LaBr₃ ·CsCl: 0.001Ce³⁺ phosphor.

2: spontaneous emission spectrum of LaCl₃ ·CsBr: 0.001Ce³⁺ phosphor;

3: excitation spectrum of LaBr₃ ·CsCl:0.001Ce³⁺ phosphor; and

4: excitation spectrum of LaCl₃ ·CsBr:0.001Ce³⁺ phosphor.

The phosphor prepared in Example 1 was excited with a light whosewavelength was varied in the range of 500-850 nm after exposure tox-rays at 80 KVp, to measure stimulation spectrum at the peak wavelengthof the emission (365 nm). The result is shown in FIG. 2.

FIG. 2 shows the stimulation spectrum of LaBr₃ ·CsCl: 0.001Ce³⁺phosphor.

The phosphors prepared in Examples 1 and 2 were excited with a He-Nelaser (wavelength: 632.8 nm) after exposure to X-rays at 80 KVp, tomeasure stimulated emission spectra. The results are shown in FIG. 3.

In FIG. 3, Curves 1 and 2 correspond to the following spectra:

1: stimulated emission spectrum of LaBr₃ ·CsCl: 0.001Ce³⁺ phosphor; and

2: stimulated emission spectrum of LaCl₃ ·CsBr: 0.001Ce³⁺ phosphor.

Further, the phosphors prepared in Examples 1 to 3 were excited with theHe-Ne laser after exposure to X-rays at 80 KVp, to measure the intensityof stimulated emission. The results are set forth in Table 1.

                  TABLE 1                                                         ______________________________________                                                     Relative Intensity of                                            Example      Stimulated Emission                                              ______________________________________                                         1           100                                                              2            95                                                               3            50                                                               ______________________________________                                    

EXAMPLE 4

The procedure of Example 1 was repeated except for varying the amount ofcesium chloride within a range of 0-10.0 mols per 1 mol of lanthanumbromide, to obtain a variety of powdery cerium activated lanthanumbromide phosphors (LaBr₃ ·aCsCl:0.001Ce³⁺).

The phosphors prepared in Example 4 were excited with the He-Ne laserafter exposure to X-rays at 80 KVp, to measure the intensity ofstimulated emission. The results ar shown in FIG. 4.

FIG. 4 graphically shows a relationship between the amount of cesiumchloride (a value) and an intensity of stimulated emission with respectto LaBr₃ ·aCsCl:0.001Ce³⁺ phosphor.

EXAMPLE 5

To a mixture of the powdery cerium activated lanthanum bromide phosphor(LaBr₃ ·CsCl:0.001Ce³⁺) obtained in Example 1 and a linear polyesterresin were added successively methyl ethyl ketone and nitrocellulose(nitrification degree: 11.5%), to prepare a dispersion containing thephosphor and the binder (10:1, by weight). Subsequently, tricresylphosphate, n-butanol and methyl ehtyl ketone were added to thedispersion. The mixture was sufficiently stirred by means of a propelleragitater to obtain a homogeneous coating dispersion having a viscosityof 25-35 PS (at 25° C.).

The coating dispersion was applied to a polyethylene terephthalate sheetcontaining titanium dioxide (support, thickness: 250 μm) placedhorizontally on a glass plate. The application of the coating dispersionwas carried out using a doctor blade. The support having a layer of thecoating dispersion was then placed in an oven and heated at atemperature gradually rising from 25 to 100° C.. Thus, a phosphor layerhaving a thickness of 250 μm was formed on the support.

On the phosphor layer was placed a transparent polyethyleneterephthalate film (thickness: 12 μm; provided with a polyester adhesivelayer on one surface) to combine the transparent film and the phosphorlayer with the adhesive layer.

Thus, a radiation image storage panel consisting essentially of asupport, a phosphor layer and a transparent protective film wasprepared.

EXAMPLE 6

The procedure of Example 5 was repeated except for employing the LaCl₃·CsBr:0.001Ce³⁺ phosphor obtained in Example 2 instead of the LaBr₃·CsCl:0.001Ce³⁺ phosphor, to prepare a radiation image storage panelconsisting essentially of a support, a phosphor layer and a transparentprotective film.

EXAMPLE 7

The procedure of Example 5 was repeated except for employing the YBr₃·LiCl:0.001Ce³⁺ phosphor obtained in Example 3 instead of the LaBr₃·CsCl:0.001Ce³⁺ phosphor, to prepare a radiation image storage panelconsisting essentially of a support, a phosphor layer and a transparentprotective film.

The radiation image storage panels prepared in Examples 5 to 7 weremeasured on the sensitivity (i.e., intensity of stimulated emission)when excited with a He-Ne laser (wavelength: 632.8 nm) after exposure toX-rays at 80 KVp. The results are set forth in Table 2.

                  TABLE 2                                                         ______________________________________                                        Example      Relative Sensitivity                                             ______________________________________                                         5           100                                                              6            95                                                               7            50                                                               ______________________________________                                    

I claim:
 1. A radiation image recording and reproducing methodcomprising steps of:(i) causing a cerium activated rare earth halidephosphor having the formula (I): LnX₃ ·aM^(I) X':Ce³⁺ (I)in which Ln isat least one rare earth element selected from the group consisting of Y,La, Gd and Lu; M^(I) is at least one alkali metal selected from thegroup consisting of Li, Na, K, Cs and Rb; each of X and X; is at leastone halogen selected from the group consisting of Cl, Br and I; and aand x are numbers satisfying the conditions of 0<a≦10.0 and 0<×0.2,respectively. to absorb a radiation having passed through an object orhaving radiated from an object; (ii) exciting said stimulable phosphorwith an electromagnetic wave having a wavelength within the range of500-850 nm to release the radiation energy stored therein as lightemission; and (iii) detecting the emitted light.
 2. The radiation imagerecording and reproducing method as claimed in claim 1, in which a inthe formula (I) is a number satisfying the condition of 0.1≦a≦2.0. 3.The radiation image recording and reproducing method as claimed in claim2, in which a in the formula (I) is a number satisfying the condition of0.2≦a≦1.0.
 4. The radiation image recording and reproducing method asclaimed in claim 1, in which Ln in the formula (I) is at least one rareearth element selected from the group consisting of Y and La.
 5. Theradiation image recording and reproducing method as claimed in claim 1,in which M^(I) in the formula (I) is at least one alkali metal selectedfrom the group consisting of Cs and Rb.
 6. The radiation image recordingand reproducing method as claimed in claim 1, in which each of X and X'in the formula (I) is Cl or Br, and X≠X".
 7. The radiation imagerecording and reproducing method as claimed in claim 1, in which x inthe formula (I) is a number satisfying the condition of 10⁻⁵ ≦x≦10⁻². 8.The radiation image recording and reproducing method as claimed in claim1, in which the said electromagnetic wave is one having a wavelengthwithin the range of 500-700 nm.
 9. The radiation image recording andreproducing method as claimed in claim 1, in which said electromagneticwave is a laser beam.
 10. A radiation image storage panel comprising asupport and a phosphor layer provided thereon which comprises a binderand a stimulable phosphor dispersed therein, in which said phosphorlayer contains a cerium activated rare earth halide phosphor having theformula (I):

    LnX.sub.3 ·aM.sup.I X':xCe.sup.3+                 (I)

in which Ln is at least one rare earth element selected from the groupconsisting of Y, La, Gd and Lu; M^(I) is at least one alkali metalselected from the group consisting of Li, Na, K, Cs and Rb; each of Xand X' is at least one halogen selected from the group consisting of Cl,Br and I; and a and x are numbers satisfying the conditions of 0≦a≦10.0and 0<x≦0.2, respectively.
 11. The radiation image storage panel asclaimed in claim 10, in which a in the formula (I) is a numbersatisfying the condition of 0.1≦a≦2.0.
 12. The radiation image storagepanel as claimed in claim 11, in which a in the formula (I) is a numbersatisfying the condition of 0.2≦a≦1.0.
 13. The radiation image storagepanel as claimed in claim 10, in which Ln in the formula (I) is at leastone rare earth element selected from the group consisting of Y and La.14. The radiation image storage panel as claimed in claim 10, in whichM^(I) in the formula (I) is at least one alkali metal selected from thegroup consisting of Cs and Rb.
 15. The radiation image storage panel asclaimed in claim 10, in which each of X and X' in the formula (I) is Clor Br, and X≠X'.
 16. The radiation image storage panel as claimed inclaim 10, in which x in the formula (I) is a number satisfying thecondition of 10⁻⁵ ≦x≦10⁻².